Solution target for cyclotron production of radiometals

ABSTRACT

Methods of producing and isolating  68 Ga,  89 Zr,  64 Cu,  63 Zn,  86 Y,  61 Cu,  99m Tc,  45 Ti,  13 N,  52 Mn, or  44 Sc and solution targets for use in the methods are disclosed. The methods of producing  68 Ga,  89 Zr,  64 Cu,  63 Zn,  86 Y,  61 Cu,  99m Tc,  45 Ti,  13 N,  52 Mn, or  44 Sc include irradiating a closed target system with a proton beam. The closed target system can include a solution target. The methods of producing isolated  68 Ga,  89 Zr,  64 Cu,  63 Zn,  86 Y,  61 Cu,  99m Tc,  45 Ti,  52 Mn, or  44 Sc further include isolating  68 Ga,  89 Zr,  64 Cu,  63 Zn,  86 Y,  61 Cu,  99m Tc,  45 Ti,  52 Mn, or  44 Sc by ion exchange chromatography. An example solution target includes a target body including a target cavity for receiving the target material; a housing defining a passageway for directing a particle beam at the target cavity; a target window for covering an opening of the target cavity; and a coolant gas flow path disposed in the passageway upstream of the target window.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 15/311,052 filed Nov. 14, 2016, which is a 371 application of PCT/US2015/031140 filed May 15, 2015, which claims priority to U.S. Provisional Patent Application No. 61/993,365 filed on May 15, 2014.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under DE-SC0008947 awarded by the Department of Energy. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to labeled radiopharmaceuticals.

2. Description of the Related Art

Radiometals (e.g., ⁶⁴Cu, ⁸⁹Zr, ⁶⁸Ga, ⁸⁶Y and ^(99m)Tc) play a pivotal role in nuclear medicine as therapeutic and imaging agents for radiation therapy and labeling of biologically important macromolecules like proteins, peptides and antibodies.

In the recent past, a rapid increase has been noted in both clinical and preclinical studies involving ⁶⁸Ga-labeled radiopharmaceuticals [Ref. 1-5]. This increase can be attributed to the favorable physical characteristics of ⁶⁸Ga (E_(βmax) 1.8 MeV, β⁺89%, T_(1/2)=67.7 minutes) for imaging various rapidly changing processes (proliferation, apoptosis, angiogenesis) and targets (growth hormones, myocardial and pulmonary perfusion, inflammation and infection), and to some extent, to newer, more reliable production and labeling methods [Ref. 1-5]. Gallium-68 labeled somatostatin analogs have already shown their superiority over the existing agent ¹¹¹In-DTPA-octreotide through enhanced sensitivity, specificity, accuracy and cost effectiveness for the diagnosis of patients with neuroendocrine tumors [Ref. 1, 6-9].

The clinical promise of ⁶⁸Ga-labeled radiopharmaceuticals clearly warrants growth of the supply of ⁶⁸Ga to meet the increasing demand in various nuclear medicine facilities. Presently, ⁶⁸Ga can be produced by two different approaches, (1) solid targetry [Ref. 10,11] and (2) the ⁶⁸Ge/⁶⁸Ga generator [Ref. 12]. The former requires high capital cost and expertise and specialized cyclotron facilities that accommodate solid targets, whereas, the latter is more broadly accessible in nuclear medicine facilities not equipped with an on-site cyclotron. The simplicity and lower capital cost of the ⁶⁸Ge/⁶⁸Ga generator have made it more popular among the nuclear medicine facilities with relatively lower number of requirements for ⁶⁸Ga labeled doses [Ref. 1, 12]. However, the breakthrough of trace quantities of the long-lived ⁶⁸Ge parent isotope (t_(1/2)=271 days) into the eluted ⁶⁸Ga remains a concern [Ref. 13]. Furthermore, with increasing applicability of ⁶⁸Ga-labeled radiopharmaceuticals, one can foresee a need for alternative production methods to meet the increasing demand especially for the relatively busy nuclear medicine centers having an on-site cyclotron. There have been previous attempts to produce ⁶⁸Ga using a cyclotron, initially employing a solid target method using ⁶⁸Zn electrodeposition on a copper substrate [Ref. 10, 14] and more recently using a solution target containing an enriched ⁶⁸ZnCl₂ solution [Ref. 15]. The solid target methods require a lengthy separation step, which is not optimal for short-lived isotopes like ⁶⁸Ga, as well as expensive solid target infrastructure.

Thus, needed in the art are methods and systems to extend the applicability of the solution target approach to the production of ⁶⁸Ga and other radiometals using a low energy cyclotron.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned drawbacks by disclosing improved methods and systems for preparing radionuclides (e.g., ⁶⁸Ga, ⁸⁹Zr, ⁶⁴Cu, ⁶³Zn, ⁸⁶Y, ⁶¹Cu, ^(99m)Tc, ⁴⁵Ti, ¹³N, ⁵²Mn, or ⁴⁴Sc). The methods of producing radionuclides (e.g., ⁶⁸Ga, ⁸⁹Zr, ⁶⁴Cu, ⁶³Zn, ⁸⁶Y, ⁶¹Cu, ^(99m)Tc, ⁴⁵Ti, ¹³N, ⁵²Mn, or ⁴⁴Sc) may comprise irradiating a closed target system with a proton beam. The closed target system may comprise a solution target.

This disclosure provides a solution target for production of a radionuclide from a target material. The solution target comprises a target body including a target cavity for receiving the target material; a housing defining a passageway for directing a particle beam at the target cavity; a target window foil for covering an opening of the target cavity; and a coolant flow path disposed in the passageway upstream of the target window. In one version of the present disclosure, there is provided a cyclotron solution target with improved heat transfer characteristics for production of radiometals. The target can be used in medium energy cyclotrons (11-20 MeV protons) for the production of radionuclides (e.g., ⁶⁸Ga, ⁸⁹Zr, ⁶⁴Cu, ⁶³Zn, ⁸⁶Y, ⁶¹Cu, ^(99m)Tc, ⁴⁵Ti, ¹³N, ⁵²Mn, or ⁴⁴Sc). The target has improved thermal transfer characteristics needed for cooling of the target solution. The target includes a water-cooled insert and a helium cooled region between the cyclotron vacuum and the solution. In one embodiment, a first foil separates the cyclotron vacuum from the helium flow region and a second foil (e.g., 20-30 microns) separates the helium region from the target solution. The geometry of the target insert is designed to maximize the heat transfer from the target solution to the cooling water.

In another aspect, the disclosure provides a solution target system for production of a radionuclide from a target solution. The system comprises a target body including a target cavity; a housing defining a passageway for directing a particle beam at the target cavity; a target solution distribution valve in fluid communication with the target cavity; a first source of a first target solution, the first source being in fluid communication with the target solution distribution valve; and a second source of a second target solution, the second source being in fluid communication with the target solution distribution valve, wherein the target solution distribution valve is movable from a first distribution position in which the first target solution flows into the target cavity to a second distribution position in which the second target solution flows into the target cavity. In one form, the system provides a target solution loading and unloading system that can select from up to six different metal solutions as reagents. There may also be up to six different delivery lines to minimize cross-contamination of isotopes.

In yet another aspect, the disclosure provides a method for producing a solution including a radionuclide, the method comprising bombarding a target solution with protons to produce a solution including a radionuclide, wherein the radionuclide is selected from ⁶⁸Ga, ⁸⁹Zr, ⁶⁴Cu, ⁶³Zn, ⁸⁶Y, ⁶¹Cu, ^(99m)Tc, ⁴⁵Ti, ¹³N, ⁵²Mn, and ⁴⁴Sc.

The foregoing and other aspects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings that form a part hereof, and in which there is shown by way of illustration certain embodiments of the invention. Such embodiments do not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an exploded cross-sectional view of a solution target for production of a radionuclide from a target material according to one embodiment of the present invention.

FIG. 1B is a cross-sectional view of the solution target of FIG. 1A in an assembled state.

FIG. 2 is a schematic of an automated system for the separation of ⁶⁸Ga radioisotope from a ⁶⁸Zn(NO₃)₂ target solution.

FIG. 3 is a plot showing the radionuclidic purity of ⁶⁸Ga two hours after end of bombardment (EOB).

FIG. 4 is a plot showing the radionuclidic purity of ⁶⁸Ga thirty-six hours after EOB.

FIG. 5 is an HPLC chromatogram of purified ⁶⁸Ga, with retention time of 2.62 minutes.

FIG. 6 shows a schematic diagram of ⁸⁹Zr production using a remotely controlled loading and unloading system.

FIG. 7 shows trends of gas evolution rate for chloride and nitrate solutions under proton irradiation.

FIG. 8 shows gas evolution rates as a function of (a) concentration of chloride salt; (b) cation at equal chloride content; (c) concentration of nitrate salt; (d) cation at equal nitrate content (data from Tables 5-8).

FIG. 9 shows the characterization of radionuclidic purity of a ⁸⁹Zr sample on a HPGe detector.

FIG. 10 shows a HP—Ge spectrum of a ⁸⁹Zr sample showing absence of peaks other than those due to ⁸⁹Zr.

Like reference numerals will be used to refer to like parts from Figure to Figure in the following description of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a solution target for production of a radionuclide from a target material. The solution target comprises a target body including a target cavity for receiving the target material; a housing defining a passageway for directing a particle beam (e.g., protons) at the target cavity; a target window foil for covering an opening of the target cavity; and a coolant flow path disposed in the passageway upstream of the target window. By “upstream”, we mean situated in the opposite direction from that in which the particle beam flows. The coolant flow path can decrease an energy value of the particle beam to 15 MeV or less. The solution target may include an energy degrading foil disposed within the passageway and upstream of the coolant flow path. The energy degrading foil may comprise a cobalt alloy. The target window foil may comprise an aluminum alloy.

The solution target may comprise a target solution port in fluid communication with the target cavity. The solution target may further comprise a pressure source in fluid communication with the target cavity. The pressure source may comprise helium gas. The coolant flow path may comprise helium gas, and the helium gas may be confined in a tubular conduit.

In the solution target, the target body may further include a second coolant flow path disposed adjacent to the target cavity. The second coolant flow path is defined by a space between a first target body component and a second target body component. The second coolant flow path may comprises water. The target cavity may defined by a conical wall. The target material may comprise a solution of a metal.

Referring to FIG. 1A and FIG. 1B, a solution target 10 according to one non-limiting embodiment of the invention is shown. The solution target 10 includes a target body 14 having a first target body component 16 and a second target body component 18. A target cavity 22 is defined by a wall 24 and is disposed within the first target body component 16. The target cavity 22 is configured to receive a target material (e.g. a target solution). In some forms, the wall 24 of the target cavity 22 may be a conical wall. The wall 24 may comprise tantalum or niobium.

The solution target 10 further includes a housing 26 configured to define a passageway 28 for a particle beam (e.g., a proton beam). The housing 26 may include a first housing section 30 having an inlet 32 for the particle beam and a portion of the passageway 28 disposed within. A second housing section 36 is spaced between the first housing section 30 and the first target body component 16 and includes a second portion of the passageway 28. A sealing member 38, for example an O-ring, can be disposed between the first housing section 30 and the section housing section 36 to provide a sealed housing 26.

A coolant flow path 40 is disposed within the second portion of the passageway 28. The coolant flow path 40 can be confined within a tubular structure, such that the particle beam flows around the tubular structure. The coolant flow path 40 may be a helium cooling system configured to degrade the energy of the particle beam to 15 MeV or less.

The solution target 10 further includes a coolant housing 46 configured to define a second coolant flow path. The coolant housing 46 includes an inlet 48 configured to receive a coolant, for example, water. Additionally, the coolant housing 46 includes two outlets 50 configured to expel the coolant.

The solution target 10 further includes a first material housing component 60 and a second material housing component 62. A target material inlet 64 is disposed within the first material housing component 60 and is in fluid communication with a target material passageway 66 and the target body cavity 16. An overpressure source 68 is disposed within the second material housing component 62 and is in fluid communication with the target material passageway 66 and the target body cavity 16.

The solution target 10 further includes a target window foil 70 disposed between the second housing section 36 and the target body 14. The target window beam entry foil 70 is configured to cover an opening of the target body cavity 16. The target window foil 70 may include a cobalt alloy, and may be configured to degrade the energy of the particle beam. In one example, the target window foil 70 is Havar®. Havar® is a heat treatable cobalt base alloy that provides very high strength, such as an ultimate tensile strength of 330,000 psi when cold rolled and heat treated. The alloy provides excellent corrosion resistance and is non-magnetic. The nominal composition is: cobalt 42.0%, chromium 19.5%, nickel 12.7%, tungsten 2.7%, molybdenum 2.2%, manganese 1.6%, carbon 0.2%. and balance iron.

In some forms, an energy degrading foil 74 may be disposed within the passageway 28 and between the first housing section 30 and a second housing section 36. The energy degrading foil 74 may be a metallic foil, for example an aluminum or aluminum alloy foil.

In operation, the particle beam (e.g., proton beam) is introduced into the passageway 28 through the inlet 32. The particle beam follows a particle beam path 80 through the first housing section 32 and the energy degrading foil 74. The partial beam 80 continues through the coolant flow path 40 and through the target window foil 70 into the target cavity 22.

The target material (e.g., a solution of a metal) is introduced into the target material inlet 64 and follows a target material path 84 through the target material passageway and into the target cavity 22. The target material disposed within the target cavity 22 is bombarded with the particle beam to create a radionuclide. The bombarded target material may be removed using material path 84.

A coolant is introduced from a source of coolant (e.g. water) into the coolant housing 46 via the inlet 48. The coolant follows a coolant path 88 from the inlet 48 into a space between the first target body component 16 and the second target body component 18. The coolant path 88 travels adjacent to the wall 24 of the target cavity 22 and through the coolant housing 46 to the two outlets 50.

The invention further provides a solution target system for production of a radionuclide from a target solution. The solution target system includes a target body including a target cavity; a housing defining a passageway for directing a particle beam at the target cavity; a target solution distribution valve in fluid communication with the target cavity; a first source of a first target solution wherein the first source is in fluid communication with the target solution distribution valve; and a second source of a second target solution, wherein the second source is in fluid communication with the target solution distribution valve. The target solution distribution valve is movable from a first distribution position in which the first target solution flows into the target cavity to a second distribution position in which the second target solution flows into the target cavity.

The solution target system may further include a target loading/unloading valve in fluid communication with the target solution distribution valve and the target cavity, wherein the target loading/unloading valve is movable from a first loading position in which at least one of the first target solution and the second target solution flows into the target cavity and a second unloading position in which a radionuclide solution flows from the target cavity to a collection vessel. The solution target system may further include an injection valve in fluid communication with the target solution distribution valve and the target cavity, wherein the injection valve holds a sample of at least one of the first target solution and the second target solution before the sample flows into the target cavity. The solution target system may further include a source of pressurized gas in gas communication with the top entry (62) to target cavity. The solution target system may further include a pressurized gas delivery valve in gas communication with the source of pressurized gas and the target cavity, wherein the pressurized gas delivery valve controls delivery of pressurized gas to the target cavity. The solution target system may further include a pressure regulating valve in gas communication with the target cavity, wherein the pressure regulating valve controls entry of pressurized gas into the target cavity. The solution target system may further include a pressure monitor in gas communication with the target cavity, wherein the pressure monitor provides a means to measure pressure within the target and may also be used to control the pressure regulating valve.

In the solution target system, a target window foil may cover an opening of the target cavity; and a coolant gas flow path may be disposed in the passageway upstream of the target window. An energy degrading foil may be disposed within the passageway and upstream of the coolant flow path.

The first target solution may comprise a first solution of a metal and the second target solution may comprise a second solution of a metal wherein the first solution of a metal is different from the second solution of a metal.

The solution target system may further include one or more additional sources of an additional target solution. Each of the additional sources is in fluid communication with the target solution distribution valve. The target solution distribution valve is movable to one or more additional distribution positions in which each of the additional target solutions flows into the target cavity.

The invention further provides a method for producing a solution including a radionuclide. The method includes the step of bombarding a target solution with protons to produce a solution including a radionuclide, wherein the radionuclide is selected from ⁶⁸Ga, ⁸⁹Zr, ⁶⁴Cu, ⁶³Zn, ⁸⁶Y, ⁶¹Cu, ^(99m)Tc, ⁴⁵Yi, ¹³N, ⁵²Mn, and ⁴⁴Sc. The target solution may be bombarded with protons in a target cavity of a solution target operating as a closed system. The method does not include a target dissolution step. The target cavity may be cooled with a coolant during the bombardment. The target cavity is maintained at a pressure below 150 psi. or below 75 psi, during the bombardment. The solution target may have a volume capacity of two milliliters or less. The target cavity may be defined by a generally conical wall and a target window foil.

In one version of the method, the target solution is yttrium nitrate and the radionuclide is ⁸⁹Zr. In another version of the method, the target solution is ⁶⁸Zn-enriched zinc nitrate and the radionuclide is ⁶⁸Ga. In another version of the method, the target solution comprises ⁶³Cu-enriched copper (II) nitrate and the radionuclide is ⁶³Zn. In another version of the method, the target solution comprises ⁸⁶Sr-enriched strontium nitrate and the radionuclide is ⁸⁶Y. In another version of the method, the target solution comprises ⁶⁴Ni-enriched nickel nitrate and the radionuclide is ⁶⁴Cu. In another version of the method, the target solution comprises scandium nitrate and the radionuclide is ⁴⁵Ti. In another version of the method, the target solution comprises ⁵²Cr-enriched or natural chromium nitrate and the radionuclide is ⁵²Mn. In another version of the method, the target solution comprises ⁴⁴Ca-enriched or natural calcium nitrate and the radionuclide is ⁴⁴Sc. In another version of the method, the target solution comprises ⁶¹Ni-enriched or natural nickel nitrate and the radionuclide is ⁶¹Cu via deuteron irradiation. The target solution may further comprise a dilute solution of nitric acid. The concentration of nitric acid in the target solution may be 0.001 M to 2.5 M, or 0.2 to 2 M. In another version of the method, the target solution comprises aqueous ethanol and the radionuclide is ¹³N.

The method may further include the steps of passing the solution including the radionuclide through a column including a sorbent to adsorb the radionuclide on the sorbent; and eluting the radionuclide off the sorbent. The term “column”, as used herein, refers to a separation technique in which the stationary bed is within a cartridge. The particles of the solid stationary phase or the support coated with a liquid stationary phase, such as resin or sorbent, may fill the whole inside volume of the cartridge (packed column) or be concentrated on or along the inside cartridge wall leaving an open, unrestricted path for the mobile phase in the middle part of the cartridge (open tubular column). Differences in rates of movement through the medium are calculated to different retention times of the sample.

The radionuclidic purity of the radionuclide can be greater than 99%, preferably greater than 99.9%, after eluting the radionuclide off the sorbent. The method may further include the step of passing the eluted radionuclide through a second column including a sorbent.

The invention further provides a disposable, single-use kit for isolation of a radionuclide from a solution including the radionuclide wherein the solution is produced by bombarding a target material in a target cavity of a solution target with protons. The kit includes a chromatographic column including a sorbent to adsorb the radionuclide on the sorbent, wherein the radionuclide is ⁶⁸Ga, ⁸⁹Zr, ⁶⁴Cu, ⁸⁶Y, ⁶³Zn, ⁶¹Cu, ^(99m)Tc, ⁴⁵Ti, ⁵²Mn or ⁴⁴Sc. In one form, the sorbent comprises a hydroxamate resin. The kit may include an eluent for eluting the radionuclide off the sorbent. The eluent may comprise a phosphate. The kit may include a radionuclide product vessel for receiving eluted radionuclide from the column. The kit may include a first fluid conduit for placing the column in fluid communication with the target cavity, and a second fluid conduit for placing the column in fluid communication with the radionuclide product vessel. The kit may include a second chromatographic column including a sorbent to adsorb impurities in an eluent used for eluted radionuclide from the column.

The invention further provides a synthesis system that employs the kit of the invention for performing the process of isolation of the radionuclide from the solution. The synthesis system may be automated using a controller. The controller can execute a stored program to: (i) deliver the solution including the radionuclide from the target cavity to a collection vessel, (ii) deliver the solution including the radionuclide from the collection vessel to the column, (iii) thereafter deliver an eluent to the column, and (iv) thereafter deliver the eluted radionuclide to a radionuclide product vessel.

EXAMPLES

The following Examples are provided in order to demonstrate and further illustrate certain embodiments and aspects of the present invention and are not to be construed as limiting the scope of the invention.

Example 1 Cyclotron Production of ⁶⁸Ga Via the ⁶⁸Zn(p,n)⁶⁸Ga Reaction in Aqueous Solution Overview of Example 1

Example 1 extends the applicability of the solution target approach to the production of ⁶⁸Ga using a low energy (<20 MeV) cyclotron. Since the developed method does not require solid target infrastructure, it offers a convenient alternative to ⁶⁸Ge/⁶⁸Ga generators for the routine production of ⁶⁸Ga. A new solution target with enhanced heat exchange capacity was designed and utilized in Example 1 with dual foils of aluminum (0.20 mm.) and Havar® cobalt alloy (0.038 mm.) separated by helium cooling to degrade the proton energy to ˜14 MeV. The water-cooled solution target insert was made of tantalum and its solution holding capacity (1.6 mL) was reduced to enhance heat transfer. An isotopically enriched (99.23%) 1.7 M solution of ⁶⁸Zn nitrate in 0.2 N nitric acid was utilized in a closed target system. After a 30 minute irradiation at 20 μA, the target solution was unloaded to a receiving vessel and the target was rinsed with 1.6 mL water, which was combined with the target solution. An automated module was used to pass the solution through a cation-exchange column (AG-50W-X8, 200-400 mesh, hydrogen form) which efficiently trapped zinc and gallium isotopes. ⁶⁸Zn was subsequently eluted with 30 mL of 0.5 N HBr formulated in 80% acetone without any measurable loss of ⁶⁸Ga. ⁶⁸Ga was eluted with 7 mL of 3 N HCl solution with 92-96% elution efficiency. The radionuclidic purity was determined using an HPGe detector. Additionally, ICP-MS was employed to analyze for non-radioactive metal contaminants. The product yield was 192.5±11.0 MBq/μ·h decay-corrected to EOB with a total processing time of 60-80 minutes. The radionuclidic purity of ⁶⁸Ga was found to be >99.9%, with the predominant contaminant being ⁶⁷Ga. The ICP-MS analysis showed small quantities of Ga, Fe, Cu, Ni and Zn in the final product, with ⁶⁸Ga specific activity of 5.20-6.27 GBq/μg. Depending upon the user requirements, ⁶⁸Ga production yield can be further enhanced by increasing the ⁶⁸Zn concentration in the target solution and extending the irradiation time. In summary, a simple and efficient method of ⁶⁸Ga production was developed using low energy cyclotron and a solution target. The developed methodology offers a cost-effective alternative to the ⁶⁸Ge/⁶⁸Ga generators for the production of ⁶⁸Ga.

Introduction to Example 1

Our initial attempts to reproduce the solution target method described by Jensen et al. [15] showed that irradiation of aqueous ⁶⁸ZnCl₂ solutions resulted in rapid pressure increase due to radiolysis-mediated release of hydrogen and oxygen. Additionally, we noticed a pinhole in the Havar® cobalt alloy target window foil, which may be related to a reaction of the Havar® cobalt alloy foil and with the ZnCl₂ solution. We have recently performed an extensive study on the mechanistic aspects of water radiolysis during production of ⁸⁹Zr in a solution target [Ref. 16-17]. This study showed that the use of nitrate salts in dilute nitric acid solutions dramatically decreased rates of water radiolysis during radiometal production [Ref. 17]. Eliminating the use of ZnCl₂ could also prolong the life of the Havar® cobalt alloy foil. In the present work of Example 1, we extended our solution target approach to the production of ⁶⁸Ga using a 1.7 M solution of zinc nitrate (isotopically enriched) in 0.2N nitric acid for the production of ⁶⁸Ga in a closed solution target system using 20 μA beam current over 30 minute proton irradiation. We also report, an automated separation of ⁶⁸Ga from ⁶⁸Zn using cation-exchange resin.

Materials and Methods for Example 1 Targetry Details

In this study we designed and developed a new solution target having reduced solution capacity (1.6 mL) with a tantalum target body insert having dual foils of aluminum (0.20 mm) and Havar® cobalt alloy (0.038 mm) separated by helium cooling to degrade the proton energy to ˜14 MeV. The new conical shape target showed enhanced heat exchange capacity, resulting in reduced gas formation during irradiation that enabled it to be run as a closed system, pressurized at ˜40 psi with oxygen. The design of the target is depicted in FIGS. 1A and 1B and was described above. A semi-automated target loading and unloading system was utilized as described in our previous study [Ref. 17]. In this study of Example 1, 30-minute irradiations were performed with 1.7 M solutions of ⁶⁸Zn nitrate (99.23% isotopic enrichment) in 0.2 N nitric acid. The proton beam current was 20 μA.

Chemicals

Zn-68 (99.23%) enriched metal was purchased from Cambridge Isotopes Laboratory (Tewksbury, Mass., USA). Hydrochloric acid (34-37% as HCl) and nitric acid (67-70% as HNO₃) both trace metal basis were purchased from Fisher Scientific (Suwanee, Ga., USA). Hydrobromic acid (48% as HBr) and acetone were purchased from Sigma-Aldrich (St. Louis, Mo., USA). AG-50W-X8 and Chelex-100 (50-100 mesh sodium form) resins were purchased from Bio-Rad (Hercules, Calif., USA).

Instrumentation

High-purity germanium gamma spectrometer (Canberra, Meriden, Conn., USA) counters running Genie 2000 software was utilized to measure the radionuclide purity. The activity readings were measured using a CRC dose calibrator (#416 setting, CRC-55tPET, Capintec, Ramsey, N.J., USA). A Dionex cation analysis HPLC system equipped with an IonPac CSSA analytical column (4×250 mm, Dionex) and in-line radioactivity detector (Carroll and Ramsey Associates, Berkeley Calif., USA) was employed to analyze for non-radioactive metal contaminants. The flow rate of mobile phase (Dionex MetPac Eluent) was 1.2 mL/min. For anion analysis, a Dionex ICS-2100 Ion Chromatography System by Thermo Scientific was used, employing an IonPac AS19 analytical column with ion suppression, and mobile phase of 70 mM KOH. A Perkin Elmer ELAN DRC II ICP mass spectrometer by Perkin Elmer was also employed to measure trace metal contaminants.

Method for ⁶⁸Ga Separation, Determination of Specific Activity and Recovery of ⁶⁸Zn(NO₃)₂

Separation of ⁶⁸Ga from ⁶⁸Zn(NO₃)₂:

An in-house built automated system was developed for the separation of ⁶⁸Ga radioisotope from the ⁶⁸Zn(NO₃)₂ target solution as outlined in FIG. 2. After irradiation, the target dump (1.6 mL) was received in a collection vial along with a 1.6 mL water rinse of the target. The rinse and dump solutions were mixed and passed through a cation-exchange column (AG-50W-X8, 1.3 g, 200-400 mesh, hydrogen form), which was pre-conditioned by washing with 15 mL of water followed by air. Both ⁶⁸Ga and ⁶⁸Zn were effectively trapped on the cation exchange column. After trapping, the column was washed with 5.0 mL of water (chelexed water to avoid any metal contamination) to remove short-lived ¹¹C and ¹³N isotopes. A cation-exchange guard column (AG-50W-X8, 5.0 g, 200-400 mesh, hydrogen form) was placed before the waste bottle to trap any potential breakthrough of ⁶⁸Zn during purification. The cation-exchange column was placed inside the dose calibrator to monitor the total radioactivity trapped on the column. A programmable microprocessor-based controller can be in electrical communication with the valves V of the system 300 of FIG. 2 to open and close the valves when necessary to transfer fluids in the fluid lines L of FIG. 2. Suitable timing of valve opening and closing can be programmed in the controller.

Zinc-68 was eluted from the column using 30 mL of 0.5 N HBr in 80% acetone solution and collected in a separate recovery vial followed by a 3 mL water rinse to remove any remaining HBr-acetone. Finally, ⁶⁸Ga was eluted with 3 N HCl (7 mL) to a product vial.

Recovery of ⁶⁸Zn(NO₃)₂ after irradiation was achieved by evaporating the 0.5 N HBr in 80% acetone solution to dryness using a rotary evaporator followed by re-dissolving the residue in concentrated nitric acid (3-5 mL) and evaporating to dryness again on the rotary evaporator. This process was repeated a total of three times to ensure the complete conversion of ⁶⁸Zn to ⁶⁸Zn(NO₃)₂. After conversion, the identity of anionic species was confirmed as nitrate using an HPLC anion chromatography method (Dionex ICS-2100 Ion Chromatography System). The HPLC method did not show the presence of any other anion. The obtained salt was still a viscous material, which on freeze-drying obtained as a solid.

Specific Activity and Trace Metal Analysis

Specific activity (GBq/μg) of ⁶⁸Ga was measured by estimating the total Ga metal present in the final product after purification. The other metal contaminants including Zn, Fe, Cu, Ni, and Ga were also analyzed using an ICP-mass spectrometer.

Measurement of Radionuclide Purity

For the measurement of radionuclidic purity we used a gamma ray spectrometer (Canberra, Meriden, Conn., USA, DSA1000) equipped with a high-purity germanium (HPGe) detector.

Results and Discussion for Example 1 Initial Experiments, Separation and Automation

Based on the preliminary study by Jensen et al [Ref. 15], our first irradiations used 1.7 M ⁶⁸ZnCl₂ in the solution target system. At a beam current of 20 ρA, the target pressure rose rapidly to >150 psi within 5 minutes, and the run had to be aborted. This result was not surprising, as we had observed similar results in our previous studies with yttrium chloride [Ref. 16]. Based on our previous results, we switched to ⁶⁸Zn(NO₃)₂ in 0.2 N nitric acid solution, pressurized with oxygen at ˜40 psi. Furthermore, due to the enhanced cooling of the new target design, the target was operated as a closed system. Under these conditions, the target was irradiated for 30 minutes at 20 μA. The in-target pressure was maintained below 100 psi over the run.

To accomplish the separation of ⁶⁸Ga from ⁶⁸Zn, we modified the cation-exchange method developed by Strelow [Ref. 18] in the early 1980s for the separation of non-radioactive Ga salts from zinc and other metal salts. A column of 1.3 g of AG-50W-X8 resin was used. ⁶⁸Ga was trapped at more than 99% efficiency. We did not observe loss of ⁶⁸Ga during the elution of ⁶⁸Zn with 0.5 N HBr in 80% acetone or during subsequent rinsing of the column with water. Finally, ⁶⁸Ga was eluted using 3 N HCl solution (7 mL) with 92-96% elution efficiency. The elution efficiency can be increased by increasing the amount of 3 N HCl solution, but at the cost of increasing the volume of acid in the subsequent processing steps. The isotope separation process was automated using an in-house radiochemistry module with actuated valves.

Production conditions of the solution target were 1.7 M ⁶⁸Zn(NO₃)₂ in 0.2 N HNO₃ solution at 20 μA beam current over a 30 minute irradiation. The production yield of ⁶⁸Ga was found to be 192.5±11.0 MBq/μA·h after isotope separation having specific activity in the range of 5.20-6.27 GBq/μg (see Table 1). These values are decay-corrected to end of bombardment (EOB).

TABLE 1 Yields of ⁶⁸Ga in solution target (n = 3) ICP-MS determined Uncorrected Corrected Yield Specific Specific quantities of Ga and other Yield Yield (MBq/ activity activity trace metals in product* (GBq) (GBq) μA · h) (GBq/μg) (mCi/μg) (μg, n = 2) 0.96 ± 0.10 1.93 ± 0.11 192.5 ± 11.0 5.20-6.27 140-169 Ga (0.32, 0.39), Fe (17.0, 95.1), Cu (51.4, 12.6), Ni (3.5. 33.7), Zn (1.9. 1.9) *these amounts were estimated for the total 7 mL of the product solution.

At 2 hours after EOB, a purified sample of ⁶⁸Ga was subjected to HPGe spectrometry and only two peaks were evident, at 511 keV and 1077 keV both corresponding to ⁶⁸Ga. Therefore, the radionuclidic purity of ⁶⁸Ga was >99.9% (see FIG. 3). However, when same sample was reanalyzed 36 hours after EOB additional peaks were observed corresponding ⁶⁷Ga, which on decay correction to EOB, was estimated to be <0.07% of the total ⁶⁸Ga (see FIG. 4). The Ga-67 emissions were predominated by the Ga-68 emissions at 2 hours after EOB. The presence of ⁶⁷Ga impurity can be attributed to two possible nuclear reactions: ⁶⁸Zn(p,2n)⁶⁷Ga and ⁶⁷Zn(p,n)yGa [Ref. 19, 20]. We would anticipate ˜0.1% ⁶⁷Ga impurity in the ⁶⁸Ga product for a 60 minute irradiation. Radiochemical purity, as measured by cation-exchange HPLC, was >99.9% (see FIG. 5).

The saturation yields for ⁶⁸Ga before and after the separation of isotopes were found to be 0.43±0.01 GBq/μA and 0.36±0.02 GBq/μA, respectively (see Table 2). We attempted to make a comparison with the saturation yield using the solid target method as reported by Sadeghi et al [Ref. 11]. These authors reported production yield as 5.032 GBq/μA·h, presumably without taking the isotope separation into consideration. The calculated saturation yield from their data is 35.3 GBq/μA. Thus, our yields were approximately 83 times less than the solid target yields. However, Sadeghi et al [Ref. 11] did not specify the time of isotope separation so we cannot compare isolated product yields. We would anticipate that the comparison of isolated yields would be somewhat more favorable for the solution target method because it does not require a target dissolution step.

TABLE 2 Saturation yields of ⁶⁸Ga* Before Before After After separation separation separation separation (mCi/μA) (GBq/μA)) (mCi/μA) (GBq/μA) 11.5 ± 2.8 0.43 ± 0.01 9.8 ± 0.57 0.36-0.02 *n = 3 and all values decay corrected to EOB.

Considering that the primary end-use of the ⁶⁸Ga would be in for labeling of molecular targeted peptides, we also analyzed for the presence of trace metal contamination in the final product using ICP-MS. Although very low quantities of Ga were found (0.32-0.39 μg), relatively higher amounts of Fe, Cu and Ni were observed (see Table 1). If necessary, these contaminants can be further reduced before labeling using SnCl₂/TiCl₃ and Amberchrom CG-161m resin as described by Van der Meulen et al [Ref. 21]. Furthermore, in order to achieve the optimum labeling conditions (pH) for various chelators, the final ⁶⁸Ga solution (7 mL, 3 N HCl) can be concentrated using an anion-exchange column, where [⁶⁸GaCl₄]⁻ will be effectively trapped and can be further eluted with water as previously described [Ref. 1, 22]. The final pH can be readjusted using the buffer required to meet the labeling conditions. We estimate the cost of a single dose (370 MBq) of the ⁶⁸Ga produced from a cyclotron using the ⁶⁸Zn(NO₃)₂ in 0.2 N HNO₃ solution target method to be $20-25 assuming 85-90% recovery of ⁶⁸Zn, which we found in our initial attempts.

The unoptimized separation time was in the range of 60-80 minutes, but we would anticipate reduction of this time to ˜45 min with further development. We were able to make ˜25 mCi (0.92 GBq)⁶⁸Ga at end of separation. By increasing ⁶⁸Zn nitrate concentration (2×) and beam current (2×), we anticipate that >100 mCi (3.7 GBq) can be practically achieved. This would serve the need for 2-4 subjects, depending on labeling yields.

Conclusions for Example 1

A solution target approach for production and automated separation of ⁶⁸Ga was successfully developed employing a solution of ⁶⁸Zn nitrate in 0.2 N nitric acid. The production yield was found to be 192.5±11.0 MBq/μA·h decay-corrected to EOB with a specific activity in the range 5.20-6.27 GBq/μg. Radiochemical and radionuclidic purities were both >99.9%. Increasing the target solution concentration of ⁶⁸Zn and irradiation time may further increase the production yield. The isotope separation method employed AG 50W-X8 resin eluted with a solution of 0.5 N HBr in 80% acetone to remove the zinc isotopes, followed by elution of ⁶⁸Ga in 3 N HCl. The new target design with reduced target volume (1.6 mL) and enhanced heat transfer allowed irradiation as a closed system. Gallium-68 can, therefore, be produced on a low energy cyclotron in sufficient quantities to provide a viable alternative to the ⁶⁸Ge/⁶⁸Ga generator for those facilities that have an on-site cyclotron.

Example 2 Summary of Example 2

The existing solid target production method of radiometals requires high capital and operational expenditures, which limit the production of radiometals to the small fraction of cyclotron facilities that are equipped with solid target systems. In Example 2, we develop a robust solution target method, which can be applicable to a wide array of radiometals and would be simply and easily adopted by existing cyclotron facilities for the routine production of radiometals.

We have developed a simplified, solution target approach for production of ⁸⁹Zr using a niobium target by 14 MeV energy proton bombardment of aqueous solutions of yttrium salts via the ⁸⁹Y(p,n)⁸⁹Zr nuclear reaction. The production conditions were developed, following a detailed mechanistic study of the gas evolution.

Although the solution target approach avoided the expense and complication of solid target processing, rapid radiolytic formation of gases in the target represents a major impediment in the success of solution target. To address this challenge we performed a systematic mechanistic study of gas evolution. Gas evolution was found to be predominantly due to decomposition of water to molecular hydrogen and oxygen. The rate of gas evolutions varied >40-fold depending on solution composition even under the same irradiation condition. With chloride salts, the rate of gas evolution increased in the order rank Na<Ca<Y. However, the trend was reversed with the corresponding nitrate salts, and further addition of nitric acid to the irradiating solution minimized gas evolution. At developed condition, ⁸⁹Zr was produced in moderate yield (4.36±0.48 MBq/μA·h) and high effective specific activity (464±215 MBq/μg) using the solution target approach (2.75 M yttrium nitrate, 1.5 N HNO₃, 2 hours irradiation at 20 μA).

The novel findings of Example 2 on substrate dependent, radiation-induced water decomposition provide fundamental data for the development and optimization of conditions for solution targets. The developed methodology of irradiation of nitrate salts in dilute nitric acid solutions can be translated to the production of a wide array of radiometals like ⁶⁴Cu, ⁶⁸Ga and ⁸⁶Y, and is well suited for short-lived isotopes.

Introduction to Example 2

The PET radioisotope ⁸⁹Zr has recently received growing interest due to its good imaging properties (E_(βmax) 0.9 MeV), and its ease of conjugation to proteins and antibodies using the chelator desferrioxamine [Ref. 23-25]. The close match between the physical half-life and the clearance kinetics of antibodies has created a niche for this isotope [Ref. 26,27]. ⁸⁹Zr is commonly produced via the ⁸⁹Y (p,n)⁸⁹Zr nuclear reaction wherein Y foils [Ref. 28,29], pellets [Ref. 30], sputtered materials [Ref. 31] or depositions [Ref. 32,33] are irradiated with 12.5-15 MeV protons. The solid target approach entails high capital and operational costs for target processing, limiting the production of this isotope to the small fraction of cyclotron facilities that are equipped with solid target systems. A potential way to avoid use of solid target systems is to develop a solution target method for ⁸⁹Zr production. Solution targets are easily filled and unloaded to hot cells via tubing in the same way that ¹⁸F targets are operated. Solution targets have been recently reported for the production of ⁸⁶Y, ⁶⁸Ga and ^(94m)Tc [Ref. 34,35].

The major impediment in the development of solution targets is radiolytic formation of gases and subsequent generation of extremely high-pressures in closed target systems during irradiation, a fact previously reported by our group and others [Ref. 34-38]. In most cases, the irradiation times have been limited by this rise in target pressure. We have employed the use of a backpressure regulator to slowly bleed gas from the target system [Ref. 37,38], but this has its drawbacks of potential loss of solution or water vapor from the target, and subverting the recombination of molecular hydrogen and oxygen formed by radiolysis. In Example 2, we investigated the mechanistic basis of the gas formation and how it could be minimized or even overcome to develop a robust solution target for the radiometal production.

For target development, we selected ⁸⁹Zr as target isotope based on its level of interest in the imaging community and the affordability of its yttrium reagent materials. We realized that ⁸⁹Zr would represent a worst-case scenario for solution target applications because its longer half-life (T_(1/2)=78.4 hours) would require long irradiations to produce significant amounts of radiometal and would not compare favorably to solid target production methods. Even so, the low cost for development efforts and the long irradiation requirement for ⁸⁹Zr production would provide an appropriate situation to test a solution target for longer irradiations of other interesting isotopes. We anticipated that a production methodology developed for ⁸⁹Zr could be translated to other shorter-lived isotopes such as ⁶⁸Ga, ⁶⁴Cu and ⁸⁶Y. In Example 2, we investigated the in target chemistry associated with proton bombardment of aqueous solutions of yttrium salts and developed conditions for ⁸⁹Zr production using Y(NO₃)₃ solutions.

Materials and Methods for Example 2 Targetry Details

A Bruce Technologies (Raleigh, N.C.) TS-1650 target was used (3 mL Nb insert, 0.16 mm Nb window foil) within a PETtrace cyclotron (GE HealthCare, Waukesha, Wis., USA). Incident proton energy to the solutions was ˜14 MeV. A semi-automated system was developed for target loading and unloading (see FIG. 6). The system was comprised of a syringe pump, a 2-position injection valve (V6) having a 3 mL loop to hold the target loading solution, and valves that enabled loading (V1) and unloading (V2) of the target. On the top of the target, a common line connected to a pressure monitoring transducer, a low-pressure gas line (helium/oxygen to pressurize the target through pressurized fluid delivery valve V3), and a backpressure regulator (pressure regulating valve V4). The backpressure regulator was further connected to a delay line within the cyclotron vault in order to contain radioactive gas emissions. The target of the system of FIG. 6 may be the solution target 10 of FIGS. 1A and 1B with the system of the FIG. 6 being in fluid communication with the target cavity 22 of the solution target 10.

The target was cooled with the standard chilled water system on the PETtrace with incoming water at near room temperature. For mechanistic study, beam current was set at 25 μA and irradiation time was 5 minutes. Only five minute irradiation was chosen for mechanistic study as we found 5 minute time frame was enough to make a significant difference in the rate of gas evolution with different substrates. A backpressure regulator limited target pressure to <60 psi. Evolved target gas volume was measured using a volumetric syringe.

Chemicals

Yttrium nitrate (Y(NO₃)₃.6H₂O) was purchased from Strem Chemicals (Newburyport, Mass., USA). Yttrium chloride (YCl₃.6H₂O, trace metals basis), oxalic acid dehydrate [TraceSELECT®, ≥99.9999% metals basis] sodium carbonate, sulfanilamide, sodium metabisulfite, guaiacol, triethanolamine and hydrogen peroxide solution (30 wt % in H₂O) were purchased from Sigma-Aldrich (St. Louis, Mo., USA). Diethylenetriamine-pentaacetic acid pentasodium salt solution (40% aqueous solution) and 8-Anilino-1-naphthalenesulfonic acid ammonium salt hydrate were purchased from Acros Organics. Hydrochloric acid (34%-37% as HCl) and nitric acid (67%-70% as HNO₃) both trace metal basis were purchased from Fisher Scientific (Suwanee, Ga., USA). Chelex-100 resin (50-100 mesh sodium form) was purchased from Bio-Rad. Desferrioxamine mesylate was purchased from EMD Chemicals. i-TLC paper was purchased from Agilent Technologies (Palo Alto, Calif., USA).

Instrumentation

The radioactive samples were analyzed using a Wizard 2480 gamma counter (Perkin Elmer, Waltham, Mass., USA), high-purity germanium gamma spectrometer (Canberra, Meriden, Conn., USA) counter running Genie 2000 software. The activity readings were recorded using a CRC dose calibrator (489 setting, CRC-55tPET, Capintec, Ramsey, N.J.). An Agilent Cary 60 UV-Vis spectrometer was used for light absorption measurements.

Measurement of Rate of Gas Evolution

To maintain a constant target pressure, the gas was expelled through a backpressure regulator (60 PSI, Optimize Technologies, Oregon City, Oreg., USA) and evolved gases were collected in a volumetric syringe placed at the end of the line in a shielded hot shell. To avoid inaccuracies caused by the variable period of beam tuning on the gas evolution, we started the recording of gas volumes after one minute into the irradiation. Rate of gas evolution was averaged over the remainder of the irradiation period.

Synthesis and Characterization of Y(OH)₃ by Infra-Red Spectroscopy

Yttrium hydroxide was synthesized by reacting yttrium chloride (3.3 mmol, 1.0 g) in 3.0 mL of water with sodium hydroxide (11.5 mmol, 0.46 g) also in 3.0 mL of water initially at 0° C. for 15 minutes followed by 30 minutes at room temperature. Obtained precipitate was filtered and washed with 30 mL of deionized water to remove sodium chloride, unreacted sodium hydroxide and yttrium chloride. The desired precipitate was further freeze-dried to remove traces of water. FT-IR spectroscopy was performed using a KBr pellet (Thermo-Nicolet 370 FT-IR Avatar).

Method for ⁸⁹Zr Separation and Determination of Specific Activity Separation of ⁸⁹Zr

Separation of ⁸⁹Zr radioisotope from yttrium in irradiated Y(NO₃)₃ solutions was achieved by slightly modifying the literature methods [Ref. 31, 39, 40]. After irradiation, target solution mixture was transferred onto a custom-made column consisting of 75 mg of hydroxamate-derivatized resin. The column was activated prior to the loading of ⁸⁹Zr solution by washing with 8 mL of pure acetonitrile optima grade, 15 mL of water (pH=7.0, passed through Chelex-100 resin) and 2 mL of 2 N HCl (trace metal basis grade). The original container of ⁸⁹Zr was washed twice with two 5 mL portions of 2 N hydrochloric acid (trace metal basis) and loaded onto the column to remove any residual traces of ⁸⁹Zr. This was followed by washing the column with six times 12 mL portions of 2 N HCl for a total volume of 72 mL, and subsequently washed the column with 20 mL of Chelex-treated water, followed by aspiration to reduce the retained water on the column. Finally, 1.5 mL of 1 M oxalic acid solution (trace metal basis) was used to elute purified ⁸⁹Zr oxalate.

Specific Activity

Specific activity (MBq/μg) of ⁸⁹Zr was measured by adopting the literature methods [Ref. 31, 39, 40], in gist; it was a quantitative estimation of ⁸⁹Zr by titrating ⁸⁹Zr-oxalate solution with known quantities of desferrioxamine, prepared by serial dilution. The complexation occurred fairly fast at room temperature within 60 minutes. The relative amounts of free and bounded ⁸⁹Zr were estimated by i-TLC developed in diethylenetriaminepentaacetic acid (DTPA) followed by measuring the relative ratios of γ-emission produced by ⁸⁹Zr in both bounded (complexed) and free form using gamma spectrometer.

Determination of NO₂ Content by the TGS Method

To determine the concentration of NO₂ in the gas evolved from the irradiated target we utilized the “TGS-method” as adopted by the US Environmental Protection Agency (EPA). It is a colorimetric determination assay initially described by Mulik et al. and others [Ref. 40,42]. The method utilized an NO₂ absorption solution that was composed of the following reagents: 2% triethanolamine (w/v); 0.05% guaiacol/o-methoxyphenol (w/v); and 0.25% (w/v) sodium metabisulfite in water. Expelled gas from the solution target was bubbled through 30 mL of the absorption solution kept at 0° C. After completed bubbling and absorption of NO₂, the solution was stored at 4° C. overnight for the decay of short lived radioisotopes (¹¹C and ¹³N). To 1 mL of the solution was added 100 μl of diluted hydrogen peroxide solution (1 part 30% H₂O₂ to 125 parts water), and the solution was mixed well. To this solution, 500 μl of (2.15% (w/v)) sulfanilamide in 10% HCl was added. After vigorous mixing, 600 μl of the coupling reagent (2% (w/v) ammonium 8-anilino-naphthalene sulfonate in water) was added and the solution was mixed thoroughly. Absorbance (550 nm) was determined after letting the mixture stand at room temperature for 1 minute. The absorption solution was used as a blank. Calibration standards for NO₂ concentrations were generated by bubbling specific volumes of commercially obtained 0.981% NO₂ (Praxair, Rochester, Minn., USA) to 30 mL of absorption solution. Absorbance measurements for the standards defined a linear calibration curve. To determine the effective volume of NO₂ released from the target, a linear regression was performed using the calibration data.

Determination of Chlorine (Cl₂) Content During YCl₃ Irradiation by EMD-Chlorine Kit

The efflux gas was bubbled through three 30 mL vials connected in series containing deionized water at 0° C. The solutions were stored at 4° C. overnight for the decay of the short-lived radioisotopes (¹¹C and ¹³N). The chlorine concentration (mg/mL) was measured according to the EMD-chlorine kit procedure.

Determination of Hydrogen and Oxygen Content

Evolved target gas was qualitatively analyzed for H₂ using a hydrogen detector (SRI Instruments, Torrence, Calif., USA). A Clark-type oxygen probe detector (YSI Instruments, Yellow Springs, Ohio, USA) was used to quantify oxygen content in the evolved target gas. During an irradiation of a solution of 1.7 M Y(NO₃)₃ in 1 N HNO₃, the evolved target gas was passed through a trap of 1 N NaOH before flowing through a chamber containing the oxygen probe submerged in a minimal volume of water.

Measurement of Radionuclidic Purity

For the measurement of radionuclidic purity we used a high-purity Germanium (HPGe) radiation detection spectrometer (Canberra, Meriden, Conn., USA).

Statistical Analysis

All values are given as mean±standard deviation. Statistical significance of differences in gas evolution rates were determined by two-tailed student's T-test. P values <0.05 were considered statistically significant.

Results and Discussion for Example 2 Initial Experiments and Major Impediments

During initial proton irradiations of aqueous Y(NO₃)₃ solutions (0.85-1.7 M), prolonged evolution of gases (>5 ml/min) and in-target precipitation of salts limited irradiations to less than 5 minutes at 25 μA. A backpressure regulator (0.41 MPa) was installed at the headspace of the target to allow release of gases while maintaining target pressure. The infra-red spectrum of the target precipitate was obtained and compared with that of synthesized Y(OH)₃. The Y(OH)₃ standard sample showed a broad peak at 3445 cm⁻¹ that matched with the broad peak at 3495 cm⁻¹ from the target precipitate. This confirmed the target precipitate as yttrium hydroxide. The infra-red spectra of Y(NO₃)₃ and commercially obtained Y₂O₃ were also compared with obtained precipitate but did not match (no OH stretching peak), ruling out the possibility of Y(NO₃) and Y₂O₃.

Mechanistic Study to Understand the in-Target Chemistry

We embarked on a mechanistic study of the gas evolution and in-target precipitation that might inform a more optimal production strategy. Irradiation of yttrium nitrate solutions resulted in the evolution of NO₂ as predicted from literature studies [Ref. 43,44] (see Table 3). However, only small quantities (<0.1%) of NO₂ were found. Irradiation of yttrium chloride solutions resulted in evolution of relatively small (<2.5%) fractions of Cl₂ (see Table 4), however total gas evolution dramatically increased (>39 mL/min, see Table 5). Using a hydrogen detector and Clark-type oxygen probe detector, the predominant evolved gases were characterized as H₂ and O₂, respectively. The hydrogen concentration was not quantified. During an irradiation of 1.7 M Y(NO₃)₃ in 1 N HNO₃, the evolved target gas was passed through a trap of 1 N NaOH before flowing through a chamber containing the oxygen probe submerged in a minimal volume of water. The oxygen concentration was found to be 33.3%±0.04% as consistent with the stoichiometric decomposition of water to molecular H₂ and O₂.

TABLE 3 Efflux of NO₂ gas at different time intervals during 120 min proton irradiation of 2.75M Y(NO₃)₃ solution at 20 μA beam current Rate of Amount of NO₂ Amount of NO₂ Amount of NO₂ Amount of NO₂ Total amount NO₂ Average ± sd evolved 0-30 min evolved 30-60 min evolved 60-90 min 90-120 min of NO₂ evolved evolved rate of NO₂ Run (mL) (mL) (mL) (mL) 0-120 min (mL) (mL/min) evolved (mL/min) 1 0.52 3.40 4.90 5.42 14.26 0.118 0.093 ± 0.04 2 2.01 3.86 3.86 3.82 13.57 0.109 3 0.13 0.19 0.68 4.79 5.79 0.048

TABLE 4 Estimated chlorine (Cl₂) evolution during 5 min YCl₃ irradiation. Average Rate Chlorine content Chlorine content Chlorine content Total Chlorine Total Chlorine Rate of Chlorine of Chlorine Run^(a) in Vial 1 (mg) in Vial 2 (mg) in Vial 3 (mg) content (mg) content (mL) content (mL/min) Evolution (mL/min) 1 1.2 3.6 1.2 6.0 1.9 0.38 0.33 ± 0.08 2 1.2 1.2 1.2 3.6 1.1 0.23 3 1.2 3.6 1.2 6.0 1.9 0.38 ^(a)Irradiation conditions: 1.7M YCl₃, 1N HNO₃, 5 min. 20 μA beam current.

Effect of Cation and Anion

The rate of gas evolution depended on both cation and anion solutes and their concentrations (see Tables 5-6). A solution of 1.7M YCl₃ produced 8-fold higher gas as compared to 1.7M Y(NO₃)₃. In contrast, NaCl produced one-third the rate of gas evolution compared to NaNO₃ (see Table 5). The rate of gas evolution for calcium salts lay between Y and Na salts (see Table 5). Thus, a gradual increase in the rate of gas evolution was observed for chloride salts upon descending diagonally from Na to Y in periodic table (see FIG. 7). This trend was reversed for nitrate salts and led to decreases in the rate of gas evolution from Na to Y (see Table 5). The reversed trend strongly indicates disparate interactions of metal cations (Na⁺, Ca²⁺ and Y³⁺) and anions (Cl⁻ and NO₃ ⁻) in the solution target. The effect of changing salt concentration yielded mixed results. Increasing YCl₃ concentration from 0.57 M to 1.7 M resulted in a 47% increase in gas evolution, while no effect was observed for the same increase in concentration of Y(NO₃)₃ (see Table 5).

TABLE 5 Rate of gas evolution of different salt solutions at variable concentrations Molar concentration Rate of gas evolution (mL/min)* Yttrium (M³⁺) Y(NO₃)₃ YCl₃ 0.57M 6.22 ± 0.96 26.66 ± 0.01^(a) 1.7M 5.29 ± 0.07  39.22 ± 2.26 ^(a,b) Calcium (M²⁺) Ca(NO₃)₂ CaCl₂ 0.85M 8.75 ± 0.50 10.67 ± 1.87^(c) 1.7M  8.89 ± 0.69^(d)   19.16 ± 3.58 ^(e,f) Sodium (M⁺) NaNo₃ NaCl 1.7M 18.78 ± 0.69^(g )   4.89 ± 0.51 ^(h.i) *25 μA beam current, 5 min irradiation time and n = 3 ^(a)p value < 0.002 versus equimolar Y(NO₃)₃. ^(b) p value < 0.02 versus 0.57M YCl₃. ^(c)p value < 0.05 versus 0.57M YCl₃. ^(d)p value < 0.02 versus 1.7M Y(NO₃)₃. ^(e) p value < 0.05 versus 1.7M Ca(NO₃)₂. ^(f) value < 0.003 versus 1.7M YCl₃. ^(g)p value < 0.001 versus 1.7M Y(NO₃)₃ and 1.7M Ca(NO₃)₂. ^(h) p value < 0.001 versus 1.7M NaNO₃. ^(i) p value < 0.02 versus 1.7M CaCl₂.

Effect of Nitric Acid

In an attempt to reduce the rate of gas evolution via the radiolysis mechanism, the free radical scavengers nitric acid and ethanol were employed as additives (see Tables 6, 7, 8). Addition of 1 M nitric acid to the irradiating solutions resulted in 40%, 50% and 75% reductions in the rate of gas evolution for 1.7 M solutions of Y(NO₃)₃, YCl₃ (see Table 6) and NaNO₃, respectively (see Table 7). Furthermore, 48% and 50% reductions in rate of gas evolution were observed for 2.55M solutions of Ca(NO₃)₂ and CaCl₂, respectively, on addition of 1 M nitric acid (see Table 6). Exceptionally, a 20% increase in gas evolution was observed on addition of 1 M of nitric acid to 5.1 M NaCl (see Table 6). This increase in gas evolution can be attributed to the chemical reaction of concentrated NaCl and nitric acid, which leads to the formation of HCl gas and NaNO₃ [Ref. 45]. Overall, addition of 1M nitric acid resulted in significant reductions of gas evolution. When Y(NO₃)₃ concentration was kept at 1.7 M increasing the nitric acid concentration from 1 M to 2 M gave a 67% reduction in rate of gas evolution (see Table 8), but no significant difference was observed above 2 M. Furthermore, the addition of nitric acid eliminated the precipitation of salts within the target. This may be attributed to an immediate resolubilization of Y(OH)₃ in nitric acid to Y(NO₃)₃, if indeed the former is produced in the radiation spur. Alternatively, the nitric acid may effectively scavenge the hydroxyl radicals before they interact with yttrium. The effect of nitric acid addition suggests radiolysis to be a dominant mechanism for gas evolution in the solution target. Addition of ethanol to both YCl₃ and Y(NO₃)₃ resulted in rapid formation of precipitates in the target (see Table 7), precluding meaningful interpretation of the gas evolution rates.

TABLE 6 Effect of cation and nitric acid on the rate of gas evolution at same anion concentrations. Molar concentration of the target solution Rate of gas evolution (mL/min)^(a) No Acid 1M HNO₃ No Acid 1M HNO₃ Yttrium (M³⁺) 5.29 ± 0.07 Y(NO₃)₃ 3.17 ± 0.43^(v) 39.22 ± 2.26 YCl₃ 18.72 ± 2.69^(v) 1.7M 18.72 ± .69^(v )  Calcium (M²⁺) 5.75 ± 0.79 Ca(NO₃)₂ 2.97 ± 0.29^(v) 20.00 ± 1.25 CaCl₂ 10.00 ± 2.05^(v) 2.55M 10.00 ± 2.05^(v ) Sodium (M⁺) 14.25 ± 0.43  NaNO₃  3.75 ± 0.25v 10.08 ± 0.58 NaCl 12.75 ± 1.56 5.1M ^(a)25 μA beam current, 5 min irradiation time and n + 3. ^(v)p value <0.05 versus same salt without HNO₃.

TABLE 7 Effect of ethanol on the rate of gas evolution Rate of gas Rate of gas efflux^(a) efflux^(a) Target solution (mL/min) Target solution (mL/min) 1.7M YCl₃ 39.22 ± 2.26 1.7M YCl₃ + 41.0 (ppt)(n = 1) 50 mM EtOH 1.7M Y(NO₃)₃  5.29 ± 0.07 1.7M Y(NO₃)₃ + 4.25 ± 0.63 (ppt) 50 mM EtOH 1.7M NaNO₃ 18.78 ± 0.69 1.7M NaNO₃ + 4.44 ± 0.53^(ν)     1M HNO₃ H₂O (no salt) No observed gas 1M HNO₃ 5.83 ± 0.29^(ε )   ^(a)25 μA beam current, 5 min irradiation time and n = 3, ppt - observed precipitate. ^(ν)p value < 0.05 versus 1.7M NaNO₃. ^(ε)p value < 0.001 versus pure water.

TABLE 8 Effect of nitric acid and yttrium nitrate concentration on the rate of gas production. Rate of gas Rate of gas efflux* efflux* Target solution (mL/min) Target solution (mL/min) 1.7M Y(NO₃)₃ + 3.17 ± 0.43 0.7M Y(NO₃)₃ + 3.63 ± 0.22 1.0M HNO₃ 1.5M HNO₃ 1.7M Y(NO₃)₃ +  1.75 ± 0.14^(a) 1.53M Y(NO₃)₃ +  2.11 ± 0.25^(b) 2.0M HNO₃ 1.5M HNO₃ 1.7M Y(NO₃)₃ + 1.72 ± 0.35 2.75M Y(NO₃)₃ + 1.54 ± 0.61 3.0M HNO₃ 1.5M HNO₃ *25 μA beam current, 5 min irradiation time and n = 3. ^(a)p value < 0.05 versus 1.7M Y(NO₃)₃ + 1N HNO_(3.) ^(b)p value < 0.05 versus 0.7M Y(NO₃)₃ + 1.5M HNO₃.

Colligative Properties

The colligative properties of the solutions were examined by keeping the anion concentration the same while changing the cation. When chloride is kept constant at 5.1 M, NaCl (5.1 M), CaCl₂ (2.55 M), and YCl₃ (1.7 M) showed a pattern of increasing gas evolution (Na→Y) in the absence of acid (see Table 6). This pattern of gas evolution was not observed when the nitrate ion concentrations were kept the same (see Table 6). While no difference was observed between Y and Ca, the rate of gas evolution was paradoxically higher with Na. We do not have a clear explanation of this finding, but there appears to be an interaction between the various cations and anions. Since radiolysis-induced gas evolution may be at a minimum rate with nitrate salt solutions, the effect of cation may be accentuated.

Putative Explanation of Gas Evolution

The radiolysis of water due to ionizing radiation is well documented in literature [Ref. 46,47]. Mechanistic studies of water radiolysis have used kinetics, linear energy transfer characteristics and identification of free radicals [Ref. 48-66]. The primary radiation induced decomposition products of water are hydrogen and hydroxyl radicals: H₂O

H.+HO.  (1) Eqs. (2a-d) describe the putative reactions for the formation oxygen and hydrogen gas from hydrogen and hydroxyl radicals: H.+H.→H₂  (2a) H.+H₂O→H₂+HO.  (2b) HO.+2H₂O₂→2H₂O+O₂+HO.  (2c) 2H₂O₂→2H₂O+O₂  (2d) Irradiation of pure water with protons in a cyclotron target produced minimal quantities of gas, as evidenced by a small and stable increment in target pressure and no measurable gas efflux (see Table 7). As noted previously [Ref. 60, 64-66] in this situation the steady-state rate of formation of radicals must equal the rate of recombination of radicals (Eq. (1)). The presence of metal salt in the water may alter the balance of formation and recombination of radicals and thereby promote radiolysis. Thus, the formation of any chemical species utilizing either H. or OH. radicals will reduce their rate of recombination.

Recent studies conducted by the Mostafavi group [Ref. 52-56] using picosecond pulse radiolysis on 2 M sodium chloride/bromide solutions showed the formation of Cl/BrOH.⁻ intermediates via Eqs. (3) and (4) [Ref. 62,63]. At 5-6 M halide concentrations, the predominant free radical formed is X₂.⁻ (X═Cl, Br) [Ref. 52,53]. The high yield of XOH.⁻ (X═Cl, Br) at 2 M concentrations suggests consumption of hydroxyl radicals by halide ions. We could not measure free radical intermediates in our studies, but our observations of high gas evolution rates with chloride salts, particularly of Ca and Y are consistent with this mechanism. In particular, the increase of chloride concentration as the solutes were changed from NaCl→CaCl₂→YCl₃ resulted in dramatic increases in gas evolution (see Table 5), putatively due to increase in the rate of hydroxyl radical scavenging to form the ClOH.⁻ intermediate and thus decrease H. and OH. recombination. Increasing chloride salt concentrations while maintaining the same cation also increased the rate of gas evolution (see Table 5, FIG. 8a ). HO.⁻+X⁻→XOH.⁻(X═Cl,Br)  (3) HO⁻+X.→XOH.(x=Cl,Br)  (4) Interestingly, when solutions of the chlorides of Na, Ca and Y were irradiated while maintaining the same chloride concentration (see FIG. 8b ), differences in gas evolution rates were also observed, demonstrating the role of the metal cations as well. To further support the hydroxyl radical consumption hypothesis, when 1 N HNO₃ was added to the solution of YCl₃ the rate of gas evolution went down by 50% with no precipitation observed. This can be attributed to the high affinity of the hydroxyl species with the acidic protons (H⁺), thereby forming H₂O.⁺ (also known as “hole”): HO.+HNO₃H₂O.⁺+NO₃ ⁻  (5a) According to the studies of Balcerzyk et al. [Ref. 52,53], a hole can further interact with a nitrate ion to form a nitrate radical and water molecule (Eq. (5b)). H₂O.⁺+NO₃ ⁻→NO₃.+H₂O(indirect effect-hole scavenging)  (5b) Additionally, nitrate radicals can also be formed directly by irradiation of nitrate (Eq. (6)) or through conversion of nitrate to nitrite and the oxygen radical intermediate (.O—) (Eq. (7a) & (7b)) [Ref. 51,52]: NO₃ ⁻→NO₃ .+e ⁻(direct)  (6) NO₃ ⁻→NO₂ ⁻+.O.(direct)  (7a) NO₂ ⁻+.O⁻+H⁺→NO₃.+HO.(indirect)  (7b) Finally, the nitrate radical can combine with hydrogen radical to form nitric acid, thereby competing with the formation of hydrogen gas: H.+NO₃.→HNO₃  (8)

Thus, the presence of nitric acid effectively converts hydrogen and hydroxyl radicals back to water via the combination of Eqs. (1) and (5-8), although several free radical species (NO₃., H₂O.⁺, .O—) may be involved as intermediates.

Irradiation of nitrate salts in the absence of nitric acid allows greater interaction of hydroxyl radicals with metal cations resulting in increased gas evolution. However, hydroxyl radicals would be predicted to be increased in nitrate solutions according to Eqs. (6a and b) whereas hydroxyl radicals are consumed by chloride according to Eqs. (3 and 4), consistent with the markedly higher gas evolution rates observed with chloride salts compared to the nitrate salts. Nevertheless, the presence of nitrate ion per se appears to decrease water decomposition as shown by the comparison of gas evolution rates for increasing salt concentrations (see FIG. 8c ). An increase in gas evolution was seen with increased chloride concentration (holding the cation concentration constant), while the opposite was seen for nitrate salts (see FIG. 8a versus 8 c). As noted previously, there exist disparate metal ion (Na⁺, Ca²⁺, Y³⁺) to anion (Cl⁻ and NO₃ ⁻) interactions in the different solutions (see FIG. 8a-8d ). FIG. 8 summarizes the trends found in gas evolution. Further mechanistic study may be required to elucidate the effects of different metal cations with anions in solution in response to proton irradiation. For example, pico-second pulse radiolysis experiments [Ref. 52-56] may be able to determine concentrations of critical intermediates that may provide more precise information about such interactions.

Developed Production of ⁸⁹Zr

Based on these findings, we developed the production conditions with 2.75 M Y(NO₃)₃ solutions to give ⁸⁹Zr production yield of 4.36-4.55 MBq/μA·h before isotope separation with specific activity 464±215 MBq/μg at 20 μA beam current over 1-2 hours of irradiation (see Table 9). To avoid precipitation of target salts, the HNO₃ concentration was increased from 1 M to 1.5 M as the duration of irradiation was increased from 1 hour to 2 hours.

All runs were performed with 2.75M Y(NO₃)₃ and beam current of 20 μA. Yields were before isotope separation.

Measurements of specific activity for seven runs encompassing the conditions are shown in Table 10. After 24 hours post-EOB (end of bombardment), a purified sample of ⁸⁹Zr was analyzed on an HPGe gamma spectrometer. The analysis showed two photon peaks at 511 keV and 909 keV energies, both corresponding to ⁸⁹Zr. No other peaks were observed, indicating that the radionuclidic purity of ⁸⁹Zr was >99% (see FIG. 9).

TABLE 9 Developed yields of ⁸⁹Zr in solution target. Irradiation Time HNO₃ Yield Yield (min) (M) (MBq) (MBq/μA · h) 60 (n = 3) 1.00  81.03 ± 14.4 4.55 ± 1.07 90 (n = 3) 1.25 108.04 ± 17.3 4.47 ± 0.62 120 (n = 3)  1.50 153.18 ± 15.1 4.36 ± 0.48

TABLE 10 Specific activities of purified ⁸⁹Zr Experiment^(a) 1 2 3 4 5 6 7 Average ± sd (n = 7) Specific Activity 192.4 358.9 418.1 518 666 802.9 292.3 464 ± 215 (MBq/μg) ^(a)2.75M Y(NO₃)₃ + 1.5N HNO₃., 120 min irradiation at 20 μA beam current.

Conclusion for Example 2

In summary, a solution target approach for production of ⁸⁹Zr was successfully developed. The major obstacles in implementing a solution target for ⁸⁹Zr production were identified as gas evolution due to radiation-induced water decomposition and in-target precipitation of salts. Gas evolution followed a consistent trend across Groups 1-3 metals, however, disparate interactions between the metal cations and anions (chloride versus nitrate) were observed. Gas evolution can be minimized significantly by addition of nitric acid and use of nitrate salts. The present study of Example 2 provides foundational information on the design and development of solution targets, and should be applicable for production of other radiometals such as ⁶⁸Ga, ⁶⁴Cu, ⁶³Zn, ⁸⁶Y, ⁶¹Cu, ^(99m)Tc, ⁴⁵Ti, ¹³N, ⁵²Mn, or ⁴⁴Sc. Applications for use of the radiometals include, without limitation, labeling of diagnostic and therapeutic compounds for medical purposes.

Example 3

Example 3 describes the production of ¹³N-ammonia with a solution target. For production of ¹³N-ammonia, the solution target was configured with a standard Havar® cobalt alloy window foil (40 μm thickness) without a degrader foil or helium cooling. The target was loaded with 1.6 mL 50 μM EtOH in deionized water and pressurized to 90 psi. The target was irradiated with 16.5 MeV protons (GE PETtrace) for 15 minutes at 35 μA beam current. Target pressure was stable during the run at ˜133 psi. The target liquid was unloaded to a hot cell, passing through a sterile filter (0.2 m) before collection in a product vial. The amount of ¹³N-ammonia produced was 513±29 mCi uncorrected for decay. The conventional silver target used for ¹³N-ammonia production yields approximately 330 mCi under the same irradiation conditions, however, in this case, the target is rinsed once with nonradioactive water to increase the transfer of the ¹³N-ammonia produced. Undesirable release of volatile ¹³N-labeled byproduct gases (NO_(x)) were reduced from approximately 15 mCi with the silver target to approximately 3 mCi with the new target.

Example 4 Overview of Example 4

Example 4 improves the cyclotron production yield of ⁸⁹Zr using a solution target by developing an easy and simple synthesis of the hydroxamate resin used to process the target, and investigating biocompatible media for ⁸⁹Zr elution from the hydroxamate resin.

A new solution target with enhanced heat dissipation capabilities was designed by using helium-cooled dual foils (0.2 mm Al and 25 mm Havar®) and an enhanced water-cooled, elongated solution cavity in the target insert. Irradiations were performed with 14 MeV protons on a 2 M solution of yttrium nitrate in 1.25 M nitric acid at 40-μA beam current for 2 hours in a closed system. Zirconium-89 was separated from Y by use of a hydroxamate resin. A one-pot synthesis of hydroxamate resin was accomplished by activating the carboxylate groups on an Accell™ Plus CM cation exchange resin using methyl chloroformate followed by reaction with hydroxylamine hydrochloride. After trapping of ⁸⁹Zr on hydroxamate resin and rinsing the resin with HCl and water to release Y, ⁸⁹Zr was eluted with 1.2 M K₂HPO₄/KH₂PO₄ buffer (pH 3.5). ICP-MS was used to measure metal contaminants in the final ⁸⁹Zr solution.

The new target produced 349±49 MBq (9.4±1.2 mCi) of ⁸⁹Zr at the end of irradiation with a specific activity of 1.18±0.79 GBq/μg. The hydroxamate resin prepared using the new synthesis showed a trapping efficiency of 93% with a 75 mg resin bed and 96-97% with a 100-120 mg resin bed. The elution efficiency of ⁸⁹Zr with 1.2 M K₂HPO₄/KH₂PO₄ solution was found to be 91.7±3.7%, compared to >95% for 1 M oxalic acid. Elution with phosphate buffer gave very small levels of metal contaminants: Al=0.40-0.86 μg (n=2), Fe=1.22±0.71 μg (n=3), Y=0.29 μg (n=1).

The new solution target doubled the production quantity of ⁸⁹Zr from the previously reported 153±15 MBq to 349±49 MBq for the same 2 hour of irradiation and 40 μA beam current. The new one-pot synthesis of hydroxamate resin provides a simpler synthesis method for the ⁸⁹Zr trapping resin. Finally, phosphate buffer elutes the ⁸⁹Zr from the hydroxamate resin in high efficiency while at the same time providing a more biocompatible medium for subsequent use of ⁸⁹Zr.

1. Introduction for Example 4

Zirconium-89 has gained considerable attention for labeling antibodies due to its favorable PET imaging characteristics (β⁺ _(max)-0.9 MeV, 22.7%) and a half-life (T_(1/2)-78.4 hours) that matches the biological half-life of antibodies. Several monoclonal antibodies have been labeled with ⁸⁹Zr using the bifunctional chelator desferrioxamine (DFO) and are currently under clinical investigation for diagnostic imaging and therapeutic monitoring applications. For example, ⁸⁹Zr-DFO-cetuximab is an anti-epidermal growth factor receptor monoclonal antibody labeled with ⁸⁹Zr that is being investigated for PET imaging of metastatic colorectal cancer [Ref. 67], while ⁸⁹Zr-DFO-bevacizumab is an anti-vascular endothelial growth factor-A monoclonal antibody labeled with ⁸⁹Zr that is being investigated for evaluation of lesions in patients with Von Hippel-Lindau disease [Ref. 68], renal cell carcinoma [Ref. 69], breast cancer [Ref. 70], and neuroendocrine tumors [Ref. 71]. Zirconium-89-DFO-trastuzumab is a radiolabeled anti-HER2 monoclonal antibody currently under investigation for breast cancer imaging [Ref. 72]. Various other monoclonal antibodies are being investigated for PET imaging of pancreatic, ovarian and prostate cancers [Ref. 73]. Zirconium-89 has also been investigated for cell labeling using ⁸⁹Zr-protamine sulfate [Ref. 74], ⁸⁹Zr-oxine [Ref. 75, 76], and ⁸⁹Zr-desferrioxamine-NCS ⁸⁹ZrDBN [Ref. 77]. The 78.4 hour half-life of ⁸⁹Zr allows in-vivo tracking of the labeled cells for prolonged periods of time (weeks), which is being instrumental in understanding the cell-based therapies and imaging.

Zirconium-89 can be produced using solid and liquid target methods via the ⁸⁹Y(p,n)⁸⁹Zr reaction [Ref. 78-80]. Solid target methods require higher infrastructure cost because of the expense of the target handling system and are challenging to implement in PET facilities equipped with self-shielded cyclotrons. However, the solution target approach offers a viable alternative to the solid target system that can meet the modest needs for ⁸⁹Zr at most institutions.

Production of radiometals in a solution target is challenging because of in-target salt precipitation and unstable target pressures caused by gas evolution during irradiation [see Example 2 above], but these problems have been attenuated by using nitrate salts of the target metal in dilute nitric acid solutions [see Example 2 above]. We reported a mechanistic study on the effect of solution composition on in-target chemistry [see Example 2 above], the effect of nitric acid [see Example 2 above], and a second-generation solution target (Brigham Mayo Liquid Target-2 or BMLT-2) design that enhances heat dissipation during production of ⁶⁸Ga [FIGS. 1A and 1B described above]. In the present study of Example 4, we investigated the use of this BMLT-2 target for the production of ⁸⁹Zr. A significant improvement in the ⁸⁹Zr-production yield compared to previous results [see Example 2 above]. We also developed a new method of elution of ⁸⁹Zr from hydroxamate resin that offers a biocompatible formulation of ⁸⁹Zr for preclinical and clinical studies. Finally, we also developed a short, simplified and one pot synthesis of the hydroxamate resin used for the separation of ⁸⁹Zr from the ⁸⁹Y target material.

Materials and Methods Targetry Details

A BMLT-2 target (FIGS. 1A and 1B) was employed for ⁸⁹Zr production as described previously. The target included a water-cooled, elongated Ta insert having target volume of 1.6 mL. The dual-foil design used helium cooling on a 0.2 mm aluminum degrader foil and a 0.025 mm Havar® target window foil. The target was installed on a PETtrace cyclotron (GE HealthCare, Waukesha, Wis.) delivering 16.4 MeV protons. The incident proton energy to the target solution was approximately to 14 MeV. A semi-automated system was employed as described previously for target loading and unloading [see Example 2 above].

Chemicals

Yttrium nitrate (Y(NO₃)₃.6H₂O) was purchased from Strem Chemicals (Newburyport, Mass., USA). Oxalic acid dehydrate [TraceSELECT®, ≥99.9999% metals basis] was purchased from Sigma-Aldrich (St. Louis, Mo., USA). Nitric acid (70% purified by redistillation) ≥99.999% trace metals basis as purchased from Fisher Scientific (Suwanee, Ga., USA). Chelex-100 resin (50-100 mesh, sodium form) was purchased from Bio-Rad. Accell Plus CM (300 Å, WAT 010740) weak cation exchange resin (carboxylate resin) was purchased from Waters Inc. (Milford, Mass., USA). The phosphate buffer used for ⁸⁹Zr elution was 1.2 M K₂HPO₄/KH₂PO₄ buffer (pH 3.5) prepared in-house using ≥99.999% trace metals basis K₂HPO₄ and KH₂PO₄, both purchased from Sigma-Aldrich (St. Louis, Mo., USA). Na₂HPO₄ and NaH₂PO₄≥99.999% trace metals basis were also purchased from Sigma-Aldrich (St. Louis, Mo., USA)

Instrumentation

For determining radionuclidic purity, a high-purity germanium gamma spectrometer (Canberra, Meriden, Conn., USA) counter running Genie 2000 software was used. Sample radioactivity was measured using a CRC dose calibrator (489 setting, CRC-55tPET, Capintec, Ramsey, N.J., USA). A Perkin Elmer ELAN DRC II ICP mass spectrometer was employed to analyze trace metal contaminants. IR spectra were recorded as KBr pellets using a ThermoNicolet Avatar 370 FT-IR (Waltham, Mass., USA).

Method for ⁸⁹Zr Separation and Determination of Specific Activity Synthesis of Hydroxamate Resin

The hydroxamate resin was synthesized by stirring the Accell™ Plus CM cation exchange carboxylate resin (2.00 g), methyl chloroformate (2.0 mL, 25.8 mmol) and triethylamine (2.0 mL, 14.3 mmol) in anhydrous dichloromethane (30 mL) at 0° C. for 30 minutes and then at room temperature for additional 90 minutes. The temperature of the mixture was further lowered to 0° C. before addition of hydroxylamine hydrochloride (0.6 g, 8.63 mmol) and triethylamine (2.0 mL, 14.3 mmol). The resultant mixture was stirred at room temperature for an additional 15 hours. The solvent was removed under vacuum, and cold water was poured with constant stirring into the flask containing the functionalized resin. The resin was filtered, washed extensively with water, and dried under vacuum. Total amount of hydroxamate resin recovered was 1.9-2.0 g.

Validation of Hydroxamate Resin

Hydroxamate resin was packed into an empty cartridge having plastic frit on both the sides. The resin was activated with acetonitrile (8 mL), water (15 mL), and hydrochloric acid (2 mL 2 M) as described previously [Ref. 79, 80] prior to testing the trapping efficiency. Different amounts of hydroxamate resin were evaluated for their efficiency in trapping of ⁸⁹Zr. The conversion of carboxylate group to a hydroxamate group on the resin was also characterized by infrared (IR) spectrometric analysis.

Separation of ⁸⁹Zr

Separation of ⁸⁹Zr from the irradiated Y(NO₃)₃/HNO₃ solution was achieved by trapping the ⁸⁹Zr on a custom-made hydroxamate column as described previously [Ref. 80]. The Y(NO₃)₃ target material was removed from the column by rinsing the resin with 2 M HCl (75 mL) and water (10 mL). Zirconium-89 was eluted from the resin using 1 M oxalic acid or a new elution reagent composed of 1.2 M K₂HPO₄/KH₂PO₄ (pH 3.5) (phosphate buffer). For the latter, ⁸⁹Zr elution was performed using a 2-step procedure whereby after rinsing with 2 M HCl followed by water to remove the Y target material, the ⁸⁹Zr-containing resin was first wetted with ˜0.25 mL of phosphate buffer and allowed to interact on the column for 30-min before eluting the ⁸⁹Zr with an additional 1.75 mL of phosphate buffer. The eluted ⁸⁹Zr is in the form of ⁸⁹Zr-hydrogen phosphate (⁸⁹Zr(HPO₄)₂).

Specific Activity

The specific activity (MBq/μg) of ⁸⁹Zr was calculated based on measurement of total amount of Zr present in the final ⁸⁹Zr(HPO₄)₂ solution using ICP-MS. The sample was assayed for other trace metals including Al, Ga, Fe, Cu, Ni, Zn, Co, Pb, Y, Sc, Rh, Mg, Lu, In, and Ca but only Fe, Y, Al and Zr were found to be present at detectable levels.

Statistical Analysis

All values are given as mean±standard deviation using Microsoft office excel program.

Results and Discussion Resin Synthesis and Trapping Efficiency

The previously described synthesis of the hydroxamate resin is a cumbersome process involving multiple reactions, purifications, and use of hazardous chemicals such as 2,3,5,6-tetrafluorophenol [Ref. 79, 82, 83]. To address the need for a more practical and economical preparation of hydroxamate resin we developed a simplified, one-pot synthesis method using the same starting carboxylate resin. The carboxylate group on the resin was activated by methyl chloroformate and then converted into a hydroxamate functionality using hydroxylamine hydrochloride in presence of triethylamine [Ref. 84]. The synthesis method was optimized for 2 g scale. We characterized the hydroxamate resin by both infrared spectroscopic analysis and ⁸⁹Zr trapping following the literature approach [Ref. 79]. The asymmetric and symmetric stretching frequencies of the carbonyl of (>C═O) of the carboxylate moiety appeared at 1571.6 and 1403.7 cm⁻¹. The relative intensity of both the peaks was decreased compared to the non-functionalized resin while additional peaks at 1726 cm⁻¹ and 1672 cm⁻¹, characteristic of the asymmetric and symmetric stretching frequencies of hydroxamate (—CONHOH) appeared in the hydroxamate functionalized resin compared to starting carboxylate resin [Ref. 79]. Additionally, a medium to strong band at 1550 cm⁻¹ for —N—H deformation and C—N stretching vibration for amide II bands were reported for hydroxamic acids [Ref. 85]. In our case, we also observed a strong band at 1552 cm⁻¹, confirming the presence of the hydroxamate group.

The resin was tested for ⁸⁹Zr trapping efficiency with different bed loads. A trapping efficiency of 93% was obtained with a 75 mg bed load. Higher amounts of resin (100-120 mg) increased the trapping efficiency to 96-97%, but larger buffer volumes were then needed to elute the ⁸⁹Zr, which resulted in more dilute final solutions (see below). Thus 75 mg of resin was found to be optimal to obtain a highly concentrated solution of ⁸⁹Zr-hydrogen phosphate or ⁸⁹Zr-oxalate.

Isolation ⁸⁹Zr as ⁸⁹Zr(HPO₄)₂

Oxalic acid has been extensively used for elution of ⁸⁹Zr from the hydroxamate resin [Ref. 79, 82, 83]. However, the toxicity of oxalic acid prevents this solution to be used directly in biological systems [Ref. 79]. Holland et al. [Ref. 79] described a method to convert ⁸⁹Zr-oxalate to ⁸⁹ZrCl₂ using a QMA cartridge (anion exchange resin) followed by elution with 0.9% saline solution or 1.0 M hydrochloric acid. Hydrochloric acid gave good elution efficiency but required 110° C. evaporation step prior to the further use of the ⁸⁹Zr, whereas 0.9% saline solution gave only 22-38% elution efficiency [Ref. 79]. There is, therefore, a need for an improved ⁸⁹Zr elution/formulation strategy. To address this issue we employed phosphate buffer to elute the ⁸⁹Zr from the hydroxamate resin. The elution efficiency was optimized by evaluating various concentrations of phosphate buffers (Table 11), and 1.2 M K₂HPO₄/KH₂PO₄ at pH 3.5 was found to be optimal, producing 91.7±3.7% elution efficiency using a total of 2 mL of buffer after an initial 30 minutes of equilibration on the column. Elution was performed in four 0.5 mL aliquots to provide an increased concentration of ⁸⁹Zr in one of the four fractions. For comparison to the conventional approach, a 1 M-oxalate solution (1 mL) was also utilized to elute ⁸⁹Zr. The metal contaminants were analyzed in both the ⁸⁹Zr-hydrogen phosphate and ⁸⁹Zr-oxalate solutions, and found that both solutions contain similar small amounts of Al, Fe and Y (see Table 12). Indeed, we have utilized this formulation strategy to effectively label stem cells without the need to reformulate ⁸⁹Zr eluted in oxalic acid [Ref. 77].

TABLE 11 ⁸⁹Zr elution efficiencies from hydroxamate resin as a function of eluent. Eluent Elution Efficiency 1M H₃PO₄ 67-84% 1M NaH₂PO₄ 40-90% 2M (pH 3.5) NaH₂PO₄ 73-94% 1M (pH 7.0) Na₂HPO₄/NaH₂PO₄ 41-93% 1.2M (pH 3.5) 91.7 ± 3.7% (n = 8) K₂HPO₄/KH₂PO₄,

TABLE 12 Metal impurities in the final ⁸⁹Zr product as a function of eluent. Metal contaminants in Metal contaminants in ⁸⁹Zr-hydrogen phosphate eluent* ⁸⁹Zr-oxalate eluent** Al = 0.40-0.86 μg (n = 2), Al = 2.39-5.38 μg (n = 2), Fe = 1.22 ± 0.71 μg (n = 3), Fe = 1.10-1.20 μg (n = 2), Y = 0.29 μg (n = 1) Y = 0.50 μg (n = 1) *Total 4 samples were analyzed for a wide range of metal impurities (listed in Example 4 method section) but detectable contaminants were present only in few samples and their numbers are written in the parenthesis. **Total 2 samples were analyzed for a wide range of metal impurities (listed in Example 4 method section) but detectable contaminants were present only in few samples and their numbers are written in the parenthesis.

Production of ⁸⁹Zr

The enhanced heat dissipation capacity of the BMLT-2 target (FIGS. 1A and 1B) resulted in stabilized target pressure and avoidance of in-target precipitation issues during ⁸⁹Zr production for beam currents up to 40 μA. We used 2 M yttrium nitrate in a 1.25 M nitric acid to minimize in-target precipitation. Initially, we tested different beam currents ranging from 10-40 μA by monitoring the change in-target pressure in a closed target system. Once we established that target pressure reached a steady state by 10-20 minutes of irradiation, we extended the irradiation time 2 hours. The production rate per mA was increased by approximately 12% from 3.82±0.37 MBq/μA·h to 4.36±0.60 MBq/μA·h, but due to the doubling of beam current the overall activity produced was found to be more than double in same 2 hours of irradiation from 153.18±15.1 MBq [see Example 2] to 348.88±48.7 MBq (Table 13). The specific activity of the ⁸⁹Zr was not changed significantly from 464±215 MBq/μg [see Example 2] to 1186±799 MBq/μg.

Analysis of metal contaminants in the ⁸⁹Zr product solution was carried out using ICP-MS (Table 14), and only small amounts of Al (2.28±1.23 μg), Fe (1.90±1.23 μg), and Y (0.29±0.21 μg) were found. All other metals were below detection limits.

TABLE 13 ⁸⁹Zr production rates in first and second generation solution targets Molar Decay concentration Beam Corrected Production of Y(NO₃)₃ Current HNO₃ EOB activity rate (M) (μA) (M) (MBq) (MBq/μA · h) TS-1650 target* 2.75 20 1.50 153.1 ± 15.1 3.82 ± 0.37 (n = 3)   (4.14 ± 0.4 mCi) BMLT-2 target 2.00 40 1.25 348.8 ± 48.7 4.36 ± 0.60 (n = 7)      9.4 ± 1.2 mCi) *data from Example 2.

-   -   All irradiations were performed for 2 hours. Yields were         calculated before isotope separation at end of beam.

TABLE 14 Specific activities of purified ⁸⁹Zr in first and second generation solution targets. Specific Activity of ⁸⁹Zr produced Specific Activity of ⁸⁹Zr in using the TS-1650*** solution target a BMLT-2 (FIGS. 1A and 1B) (n = 7) *(MBq/μg) target (n = 3)** (MBq/μg) 464 ± 215 1186 ± 799 *2.75M Y(NO₃)₃ in 1.5N HNO₃, 2 hours irradiation at 20 μA beam current. **2.0M Y(NO₃)₃ in 1.25N HNO₃, 2 hours irradiation at 40 μA beam current. ***A F-18 target designed by Bruce Technologies (see Example 2)

A purified sample of ⁸⁹Zr was analyzed on an HP—Ge gamma spectrometer for radionuclidic purity. The energy spectrum showed two photon peaks at 511 keV and 909 keV energies, both corresponding to ⁸⁹Zr. No other peaks were observed, indicating that the radionuclidic purity of ⁸⁹Zr was >99% (see FIG. 10).

CONCLUSIONS

In summary, the newly designed BMLT-2 target of FIGS. 1A and 1B increased the production rate of ⁸⁹Zr by approximately 12% from 3.82±0.37 MBq/μA·h to 4.36±0.60 MBq/μA·h; but since the new target increased current capacity 2-fold, the ⁸⁹Zr yield for the same 2 hour irradiation increased by more than 2-fold from 153.18±15.1 MBq to 348.88±48.7 MBq. But, the specific activity of the ⁸⁹Zr did not change significantly from 464±215 MBq/μg to 1186±799 MBq/μg. Additionally, a simple one-pot synthesis of the hydroxamate resin and a new method of elution of ⁸⁹Zr from the hydroxamate resin with a phosphate buffer were developed to improve the practicality of ⁸⁹Zr production and provide a final formulation in a more biologically acceptable medium than the previously used oxalic acid solution.

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The citation of any document or reference is not to be construed as an admission that it is prior art with respect to the present invention.

Thus, the present invention provides methods of producing and isolating ⁶⁸Ga, ⁸⁹Zr, ⁶⁴Cu, ⁶³Zn, ⁸⁶Y, ⁶¹Cu, ^(99m)Tc, ⁴⁵Ti, ¹³N, ⁵²Mn, or ⁴⁴Sc and solution targets for use in the methods are disclosed. The methods of producing ⁶⁸Ga, ⁸⁹Zr, ⁶⁴Cu, ⁶³Zn, ⁸⁶Y, ⁶¹Cu, ^(99m)Tc, ⁴⁵Ti, ¹³N, ⁵²Mn, or ⁴⁴Sc include irradiating a closed target system with a proton beam. The closed target system can include a solution target. The methods of producing isolated ⁶⁸Ga, ⁸⁹Zr, ⁶⁴Cu, ⁶³Zn, ⁸⁶Y, ⁶¹Cu, ^(99m)TC, ⁴⁵Ti, ¹³N, ⁵²Mn, or ⁴⁴Sc further include isolating ⁶⁸Ga, ⁸⁹Zr, ⁶⁴Cu, ⁶³Zn, ⁸⁶Y, ⁶¹Cu, ^(99m)Tc, ⁴⁵Ti, ¹³N, ⁵²Mn, or ⁴⁴Sc by ion exchange chromatography. An example solution target includes a target body including a target cavity for receiving the target material; a housing defining a passageway for directing a particle beam at the target cavity; a target window for covering an opening of the target cavity; and a coolant flow path disposed in the passageway upstream of the target window.

Although the invention has been described with reference to certain embodiments, one skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which have been presented for purposes of illustration and not of limitation. Therefore, the scope of the appended claims should not be limited to the description of the embodiments contained herein. 

What is claimed is:
 1. A method for producing a solution including a radionuclide, the method comprising: (a) bombarding a target solution comprising an aqueous solution of a nitrate salt of a metal in the presence of dilute nitric acid in a target cavity of a solution target with protons to produce a solution including a radionuclide, wherein the radionuclide is ⁶⁸Ga, wherein the target cavity is cooled with a coolant during the bombardment.
 2. The method of claim 1, wherein the target solution is bombarded with protons in the target cavity of the solution target operating as a closed system.
 3. The method of claim 2, wherein the solution target has a volume capacity of two milliliters or less.
 4. The method of claim 2, wherein the target cavity is defined by a generally conical wall and a target window foil.
 5. The method of claim 1, wherein the target solution comprises ⁶⁸Zn-enriched zinc nitrate.
 6. The method of claim 1 wherein the concentration of nitric acid in the target solution is 0.1 M to 2 M.
 7. The method of claim 1, wherein step (a) comprises bombarding the target solution with a particle beam having an energy of 15 MeV or less.
 8. The method of claim 1, further comprising: (b) passing the solution including the radionuclide through a column including a sorbent to adsorb the radionuclide on the sorbent; and (c) eluting the radionuclide off the sorbent.
 9. The method of claim 8, wherein the sorbent comprises a hydroxamate resin.
 10. The method of claim 8, wherein step (c) comprises eluting the radionuclide off the sorbent using a phosphate.
 11. The method of claim 8, wherein the radionuclidic purity of the radionuclide is greater than 99% after step (c).
 12. The method of claim 8, further comprising: (d) passing the eluted radionuclide through a second column including a sorbent.
 13. The method of claim 8, wherein step (c) comprises eluting the radionuclide off the sorbent at a pH of 3.5 to
 7. 